1. Field of the Invention
The present invention relates generally to a massive Multiple-Input Multiple-Output (MIMO) system operating in Frequency Division Duplex (FDD) mode and, in particular, to a method and apparatus of feeding back downlink channel state information efficiently with a limited amount of information in the massive MIMO system.
2. Description of the Related Art
As one of the key communication techniques to be adopted to the next generation mobile communication systems, studies are being conducted on the massive MIMO technique in which a base station equipped with multiple transmit antennas transmits data to a plurality of mobile stations on the basis of multi-user MIMO scheme. The transmit beamforming gain is logically in proportion to a number of transmit antennas. When exploiting the high beamforming gain of the massive MIMO system equipped with plural transmit antennas, it is possible to reduce the transmit power of each antenna. In this respect, massive MIMO is emerging as an important green communication technology.
In order to form the transmit beams of the massive MIMO system, the transmitter has to receive the downlink Channel State Information (CSI) for each downlink. In the Time Division Duplex (TDD) system, the downlink channel estimation technique based on Sounding Reference Signal (SRS) has been disclosed. However, this technique has a drawback in that uplink SRS's vulnerability to inter-cell interference degrades the channel estimation performance, resulting in reduction of system throughput.
In the legacy FDD Long Term Evolution-Advanced (LTE-A) system, the Channel State Information Reference Signal (CSI-RS) symbols are mapped to the Resource Elements (REs) on different carriers in the frequency domain and at different OFDM symbols in the time domain or multiplied by orthogonal codes to maintain orthogonality between transmit antenna ports at every downlink slot. The mobile station estimates the CSI-RS symbols mapped to the REs at the positions to measure downlink channel states associated with the respective transmit antennas. If the massive MIMO technique is adopted to the legacy MIMO system without modification, the transmitter will have to transmit CSI-RS as many as the number of the transmit antennas and thus, the receiver will have to feed back the downlink channel information as much as the number of transmit antennas.
Accordingly, the transmitter will have to allocate a few hundred of REs for CSI-RS transmission so as to reduce the number of REs for data transmission, resulting in a reduction of downlink data transmission capacity. In addition, the downlink channel state information amount to be fed back by the mobile station will increase exponentially as the number of transmit antennas increases such that the number of REs for uplink data transmission decreases, resulting in reduction of uplink data transmission capacity.